Project Summary- Project 2 Sleep apnea (SA) is a major health burden and chronic intermittent hypoxia (CIH) is a hallmark manifestation of SA. The overall goal of Project 2 aims at determine how CIH acting on key central nervous system (CNS) structures mediate sympathetic activation through the carotid body (CB). SA patients and CIH exposed rodents exhibit pronounced sympathetic nerve activation during the post-inspiratory phase of the respiratory cycle. While the Paraventricular nucleus (PVN) receives sensory input from the CB and is a major regulator of sympathetic tone. We recently discovered a neural network that mediates post-inspiratory activity in the brainstem: the post-inspiratory complex (PiCo). We test the hypothesis that PiCo and PVN are the major CNS areas that are critical for mediating CB reflex-dependent sympathetic excitation by CIH. We test this possibility using a combination of physiological, electrophysiological, and optogenetic approaches on rats and mice exposed to CIH, as well as in a mouse of model of sleep apnea, and brainstem slices. AIM 1 determines whether CIH increases excitability in PiCo. AIM 2 determines whether CIH alters the excitability of rostroventrolateral medulla sympathetic pre-motoneurons via PiCo. Experiments in AIM 3 addresses the influence of CIH on the interaction between PVN and PiCo. AIM 4 determines the functional role of PiCo and PVN in mediating the increased sympathetic drive caused by CIH. AIM 5 examines the role of PiCo and PVN in mediating increased sympathetic drive and apneas in HO-2 null mice which exhibit spontaneous sleep apnea. Major conceptual and technical innovations of Project 2 include: a) identification for role of PiCo in mediating increased sympathetic nerve activity by CIH, b) the delineation of a complete neural circuit responsible for increased sympathetic nerve activity by CIH, c) use of the state-of-the-art optogenetic approaches to determine the involvement of different neuronal circuits, and d) examination of central pre-motor circuits controlling sympathetic tone in a novel mouse model that exhibits spontaneous apneas. Members of the investigative team have long-standing experience and expertise with the proposed approaches, were the first to identify PiCo, and have an excellent track record of working together for number years as evidenced by joint publications. Successful completion of Project 2 is anticipated to establish a framework of understanding the CNS circuits causing increased sympathetic nerve activation and may lead to novel effective therapies for mitigating CB reflex- dependent sympathetic activation.